146 



CHIM.-KROID FISHES AND TIIKIR DEVELOPMENT. 



Ischyodus, the final Jurassic Chimsroid, deserves more detailed examination, 

 since its skeleton has been obtained in a condition of fair preservation in the Bava- 

 rian lithographic stone. From structural details, accordingly, this genus is known 

 to be widel}' separated from Myriacanthus or Squaloraja; and on the other hand it 

 resembled closely recent forms. It was thus similar in the shape of its head and 



Fig. 143. — Anterior dorsal fin and its supports. 

 A, Shark. (Squalid); B, Myriacanthus: C, Callorhynchus ; D, Chima?ra. 



trunk; its snout was fleshy and appears to have 

 terminated in a flap-like tip. Its dental plates, 

 however, are stouter (fig. 124) than in Chimsera, 

 and show fewer localized tritoral areas. Its dor- 

 sal spine was relatively short and robust, and the 

 frontal clasping organ is not unlike that of recent 

 Chimseroids, save that {cf. figs. 135 and 136) it 

 is more prominent and its ventral margin has a 

 more extended series of smaller denticles. In 

 the details of its skeleton, it strikingh' resem- 

 bles recent forms. One may also recall that 

 an egg-capsule, probably of this genus (of a 

 new genus, Aletodus, according to Jaekel), has 

 alread\' been referred to in the present paper, 

 p. 31. It resembles closely the capsule of the 

 recent Callorhynchus. 

 It is clear that in Ischyodus is represented the advancing line of Chima-roids, 

 for it extends from the upper (probably, indeed, from the lower) Jurassic as far as 

 the upper Chalk, even possibly into the Miocene (? /. helvetica), and is represented 

 during this interval by many species of many sizes. Some were probably as small 

 as the recent Chimara collici, others must have exceeded 3 meters in length. 



